While polenta base is baking, heat olive oil in a large skillet or saute pan over medium-high heat. Add the sausage and cook, breaking up and crumbling the meat as it cooks until it's cooked through and no longer pink, approximately 7-8 minutes. Drain on paper towels.
Reduce heat to medium. Add onion and bell pepper to drippings remaining in the pan. Cook 4-5 minutes or until softened but still have some "bite." Stir in garlic and cook briefly, 30 seconds.
Combine crushed tomatoes, Italian seasoning and crushed red pepper flakes. (This can be done right in the can using a small spoon to stir.) If using prepared marinara sauce, omit the Italian seasoning.
Spread seasoned crushed tomatoes or marinara sauce over the polenta.
Sprinkle with half the cheese.
Evenly distribute the sausage and peppers over the tomatoes/sauce.
Top with remaining cheese.
Bake 20-25 minutes or until hot, bubbling and cheese is nicely melted.
Let stand 5 minutes before serving. Garnish with fresh basil leaves if desired.